1. Field of the Invention
The present invention relates to a voltage-controlled phase shifter, that is, to a circuit that enables shifting of the phase of an input signal by a value determined by a control voltage. The present invention also relates to application of such a phase shifter to a voltage-controlled quartz oscillator.
2. Discussion of the Related Art
FIG. 1 schematically illustrates a conventional voltage-controlled quartz oscillator. This oscillator includes a phase shifter 10 controlled by a voltage V.sub..phi. and including differential inputs V+ and V- for receiving a differential voltage to be phase shifted. A reverse feedback loop includes a resistor 12 connected between the output Vo of phase shifter 10 and its inverting input V-. Quartz X is connected between inverting input V- and a supply potential, for example the low supply potential Vee. A feedback loop includes a resistor 13, of the same value as resistor 12, connected between output Vo and non-inverting input V+. A capacitor C is connected between non-inverting input V+ and potential Vee. Capacitor C is meant to compensate the effects of a parallel stray capacitance of quartz X and its value is chosen to be substantially equal to the value of the parallel stray capacitance.
For the circuit of FIG. 1 to oscillate, the voltage gain of phase shifter 10 must be greater than one. Then, if the phase shift is zero, the circuit oscillates at the fundamental frequency of quartz X. By imposing a non zero phase shift, quartz X is forced to oscillate at a value which is different from its fundamental frequency. The frequency setting range is limited to about .+-.0.025% from the fundamental frequency, which is appropriate for some applications.
FIG. 2 shows a conventional voltage-controlled phase shifter, such as that described in Valvo Berichte, volume XVIII, part 1/2, 1988, pages 18-20.
This phase shifter includes two differential stages which respectively include a pair of transistors Q1, Q2, and a pair of transistors Q3, Q4. The bases of transistors Q1 and Q4 receive a first differential component of phase control voltage V.sub..phi., while the bases of transistors Q2 and Q3 receive the second component of control voltage V.sub..phi.. The collector of transistor Q1 is connected to a high supply potential Vcc by two resistors 15 and 16 connected in series, and the collector of transistor Q2 is connected to potential Vcc by two resistors 17 and 18 connected in series. The collector of transistor Q3 is connected to the connection node of resistors 15 and 16, while the collector of transistor Q4 is connected to the connection node of resistors 17 and 18. Further, the collectors of transistors Q1 and Q2 are connected to each other by a resistor 19 and a capacitor 20 connected in series. Output voltage Vo of the phase shifter is taken at the connection node between resistor 19 and capacitor 20.
Differential stages Q1/Q2 and Q3/Q4 are respectively biased by the two output branches of an additional differential stage including a pair of transistors Q5, Q6. The collector of transistor Q5 is connected to the emitters of transistors Q1 and Q2, while the collector of transistor Q6 is connected to the emitters of transistors Q3 and Q4. Differential stage Q5/Q6 is biased by a current source 22 connected between low supply potential Vee and the emitters of transistors Q5 and Q6. The bases of transistors Q5 and Q6 respectively form the inverting input V- and the non-inverting input V+ of the phase shifter.
The operation of this phase shifter is described in above-mentioned Valvo Berichte document.
A disadvantage of this phase shifter is that its gain varies in large amounts according to the phase shift and reaches a minimum value close to one for a zero phase shift. This minimum value can become lower than one when the operating conditions vary or by the dispersion of the values of the components with respect to the desired values, whereby the oscillator stops.
Another disadvantage of this phase shifter is that the variation range of output voltage Vo is reduced, due to the presence of a significant number of stages between the node where voltage Vo is taken and low supply potential Vee. There is thus a significant probability that voltage Vo reaches its clipping limits when the gain of the phase shifter moves away from its minimum value, especially if the supply voltage is low. The use of such a phase shifter is thus impractical if it is desired to generate sinusoidal signals.
If the voltage gain of the phase shifter is decreased to limit clipping risks, the minimum gain is also decreased, thus increasing the risk of stopping the oscillator.
Still another disadvantage of the phase shifter of FIG. 2 is that the variation of the phase shift according to control voltage V.sub..phi. is not symmetrical. This phase shift decreases from 90.degree. and asymptotically tends to -30.degree.. Such a asymmetry disturbs the behavior of a reverse feedback loop.